The goal is to use a well-characterized model system to define processes governing the selection and activity of CD4+ CD25+ regulatory T cells (CD25+ Treg). The application centers on transgenic mice expressing the influenza virus PR8 hemagglutinin (HA) under the control of a variety of promoters, and co-expressing HA specific T cell receptors. Preliminary studies have shown that HA-specific T cells undergo selection to become CD25+ Treg to varying extents in these different lineages, and in some cases overt autoimmune disease (myocarditis, inflammatory arthritis) can develop. The application will determine how specificity for HA peptides directs CD25+ Treg repertoire formation and influences the ability of CD25+ Treg to prevent antigen-specific immune responses in HA Tg mice. Aim 1 will determine how interactions with self-peptides direct CD25+ Treg repertoire formation. How expression of self-peptides in different amounts and/or cell types directs CD25+ Treg selection in the thymus will be examined. In addition, how interactions with self peptides in the periphery contribute to CD25+ Treg repertoire formation will be assessed. Aim 2 will examine how T cell receptor (TCR) specificity directs the selection and function of CD25+ Treg. The specificity with which autoreactive TCRs interact with self-peptides during CD25+ Treg selection will be determined, and the specificity requirements for the activation and effector function of CD25+ Treg will also be defined. Aim 3 will evaluate how variations in the expression of self-peptides contribute to the ability of CD25+ Treg to prevent autoimmunity. Whether CD25+ Treg accumulate selectively in lymph nodes expressing tissue-specific antigens will be determined. How presentation of self-peptides at high levels by antigen presenting cells contributes to the ability of CD25+ Treg to prevent autoimmunity will also be assessed. These studies will provide fundamental insights into the mechanisms of immune tolerance, and into processes that can lead to the development of autoimmune disease. They will increase our understanding of the development and activity of CD25+ Treg, and of their potential application in novel therapeutic approaches for disease treatment. [unreadable] [unreadable]